1. Field of the Invention
The present invention relates to a drive control method for a vibration wave motor, a drive control device for a vibration wave motor, and an apparatus and an image forming apparatus equipped with a vibration wave motor and is applicable to an electrophotographic apparatus such as a printer, a copying apparatus or a facsimile apparatus in which a photosensitive drum or a transfer member such as a transfer belt or a transfer drum is rotated by means of a vibration wave motor, which constitutes drive means with satisfactory rotation accuracy.
2. Related Background Art
The vibration wave motor is, as described in Japanese Patent Application Laid-open No. 58-148682, to excite plural vibrations in a vibration member using periodic signals which have frequencies generally above the audible range, and to obtain a driving force by synthesizing such vibrations, and is capable of realizing stable rotation with a constant speed, e.g., by a driving method as disclosed in Japanese Patent Application Laid-open Nos. 63-1379, 60-176470, and 59-204477.
The vibration wave motor is, for example, provided with an annular shaped vibration member, a movable member maintained in contact with the annular vibration member by pressurizing means, and an output shaft connected to the movable member. On a face of the annular elastic member constituting the vibration member, there is adhered a piezoelectric element serving as an electromechanical energy conversion element, and frequency signals different in phase are applied to two driving elements of different positional phases formed in the piezoelectric element to excite a driving wave (for example a traveling wave formed by the synthesis of vending vibrations) in the elastic member, thereby driving, by friction, the movable member maintained in pressure contact with the driving surface of the elastic member in which such driving wave is excited, thus rotating the output shaft.
The drive control device for driving and controlling such a vibration wave motor compares the information detected from a rotation detector, such as an encoder composed for example of a photocoupler and a slit plate and adapted to rotation of the vibration wave motor or of a member driven by the vibration wave motor, with a target speed and executes feedback control for attaining the target speed by controlling for example the frequency of the signals applied to the above mentioned driving elements. The drive control is executed in a frequency range higher than the resonance frequency because the driving frequency and the revolution (rotation speed) is correlated so that the speed becomes highest at the resonance frequency and shows a very steep change in the frequency range lower than the resonance frequency but a relatively mild change in the frequency range higher than the resonance frequency. Consequently if the frequency is increased, the rotational speed becomes lower and if the frequency is decreased, the rotational speed becomes higher.
Such a vibration wave motor has been proposed as the drive source of various apparatus, for example for driving plural photosensitive drums (arranged at a predetermined pitch in the transporting direction of the recording material such as a recording sheet) in a color electrophotographic apparatus as an image forming apparatus, or an endless conveyor belt for continuously transporting the recording paper to the image transfer positions of the plural photosensitive drums.
In such prior technology, in the case of driving the photosensitive drum or the recording sheet conveyor belt using the vibration wave motor with an appreciable rotational precision, when the recording sheet enters the conveyor belt or when the recording sheet on the conveyor belt enters, after the image transfer step, the nip between the heating roller and the pressure roller of the fixing device for fixing the unfixed toner image on the recording sheet upon heating, there is generated an instantaneous variation of the load because the front end of the recording sheet receives the transporting force of the nip.
In the case of detecting the revolution of the motor or the driven member, and controlling the speed of the vibration wave motor at a predetermined speed by the drive control device, if the load is abruptly increased in the instantaneous variation of the load, the vibration wave motor increases the driving speed by a rapid decrease of the driving frequency applied to the motor, in order to increase the torque in response to the load.
Therefore the driving stability of the vibration wave motor is perturbed to deteriorate the quality of the image, and the vibration wave motor may eventually reach a frequency region where the motor cannot be driven, whereby the motor may be stopped.